Professor Richard Herrington, Head of the Natural History Museum's Earth Sciences Department, worked on the study with the University of Exeter's Dr Ben Williamson and Museum researcher Anna Morris.

Their findings were published in the prestigious journal Nature Geosciences at the beginning of February.

'Copper is an essential metal for a low-carbon, high-technology future,' says Prof Herrington.

'Our new geological tool could help mineral explorers choose the right places to test for undiscovered deposits and improve the efficiency of discovery.'

Magmatic minerals

Porphyry copper deposits form above large magma chambers several kilometres below the Earth's surface, and are extremely rare and difficult to find.

They currently provide around 75% of the copper in the world, 20% of the gold and 50% of the molybdenum, an element used in the production of high-strength steels.

Prof Herrington and Dr Williamson looked at the chemical composition of minerals from magmatic rocks that host porphyry copper deposits, and compared them with minerals from areas with no copper deposits.

They found that the minerals from rocks hosting copper contained excess levels of aluminium - a 'chemical signature' that could potentially be used to help identify porphyry copper deposits.

Testing the theory

The team conducted field tests using minerals extracted from two recently discovered copper sites in Chile: La Paloma and Los Sulfatos.

The case studies supported the theory that excess aluminium is associated with rich porphyry copper deposits, and the results allowed the team to unravel some of the reasons as to how these deposits formed.

Using electron microprobe and laser-ablation mass spectrometry systems at the Natural History Museum, the team showed that excess aluminium occurs in discrete concentric zones in the rock-forming minerals at La Paloma and Los Sulfatos, suggesting that it was formed through magmatic processes.

Excess aluminium can be linked to high concentrations of water in the magma. Because of this, the team concluded that the concentric zones of aluminium are probably a record of discrete injections of watery fluids into the magma chamber directly below the porphyry copper deposits.

The excess aluminium would exclude copper from the magmatic rock in this lower chamber, transferring copper and other metals upwards to form a porphyry copper deposit.

'This new method will add to the range of tools available to exploration companies to discover new porphyry copper deposits,' says Dr Williamson.

'Our findings provide important insights into why some magmas are more likely to produce porphyry copper deposit than others, and add to our understanding of how their parent magmatic rocks evolve.'

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